Reading the replies here, most are assuming that every candidate gets the maximum interview duration. I see a lot of arguments that if someone applies to M jobs and each requires N hours of interviews then they're spending M*N hours without knowing they'll get a job.

Usually you don't get scheduled for interviews at all unless there's a good chance that they expect you to pass the hiring bar. When you are scheduled for interviews, they don't go straight to the full gauntlet until the early interviews show promise.

So by the time someone goes through the 3 x 3 hour interviews in this example, there's probably a very good chance that they're being seriously considered as a hire.